The present invention relates generally to the field of virtualized tape storage and more particularly to logical volume copy policies.
With modern computing technology, non-volatile storage, e.g., disk storage, can be virtualized as tape storage, e.g., magnetic tape data storage, for clients using tape storage as their backend for storing data. The TS7700 series servers, by International Business Machines Corporation, are one example of server computers capable of virtualizing tape storage for clients needing to back up their tape storage data. Clients using tape storage for storing data can write the data to cache memory of a virtualization server, such as TS7700, which can synchronously copy the data to a non-volatile form of storage, such as, but not limited to, a hard disk drive. Files of virtualized data stored on a virtualization server can be known as logical volumes.
A plurality of virtualization servers can be pooled together in a group, i.e., a cluster grid, for data backup, wherein each server is called a cluster. Each cluster in the cluster grid can be assigned one of a plurality of copy modes based on priority, and a primary cluster which the client writes data to, i.e., a tape volume cache (TVC), can assign a copy mode to each cluster. The copy modes determine how data, e.g., logical volumes, is copied to clusters in the cluster grid. In a cluster grid, the configuration of copy modes assigned to the clusters can be known as the copy policy of the cluster grid.
Some examples of copy modes can comprise, but are not limited to, Synchronous (“S mode”), RUN (“R mode”) and Deferred (“D mode”). Synchronous is a copy mode where data is written to the TVC from a client and synchronously copied to another cluster in the grid which has been assigned S mode. S mode is useful for achieving a zero Recovery Point Objective (RPO) for preventing data loss, thus S mode can be considered a highest priority copy mode. RUN is a copy mode performed by the client as part of a Rewind Unload operation and can be a second highest priority copy mode. Deferred is a copy mode occurring asynchronously after the Rewind Unload operation performed by the client and can be a lowest priority copy mode.
In certain situations, data centers and cluster grids used for data backup may face the problem of being unable to achieve a zero RPO for a certain period of time, which can result in a loss of data. For example, if a cluster assigned S mode in a cluster grid is offline (due to maintenance or a hardware failure, etc.) data cannot be synchronously copied to the cluster from the TVC and zero RPO may not be met until the cluster is back online. A period of time where zero RPO cannot be met can result in a situation where data written to the TVC is not immediately backed up to another cluster and can therefore be lost.